Pt—Ni—Ir catalyst for fuel cell

ABSTRACT

Nanoporous oxygen reduction catalyst material comprising PtNiIr, the catalyst material preferably having the formula PtxNiyIrz, wherein x is in a range from 26.6 to 47.8, y is in a range from 48.7 to 70, and z is in a range from 1 to 11.4. The nanoporous oxygen reduction catalyst material is useful, for example, in fuel cell membrane electrode assemblies.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2017/056101, filed Oct. 11, 2017, which claims the benefit of U.S.Provisional Application No. 62/413165, filed Oct. 26, 2016, thedisclosure of which is incorporated by reference in its/their entiretyherein.

BACKGROUND

Fuel cells produce electricity via electrochemical oxidation of a fueland reduction of an oxidant. Fuel cells are generally classified by thetype of electrolyte and the type of fuel and oxidant reactants. One typeof fuel cell is a polymer electrolyte membrane fuel cell (PEMFC), wherethe electrolyte is a polymeric ion conductor and the reactants arehydrogen fuel and oxygen as the oxidant. The oxygen is often providedfrom the ambient air.

PEMFCs typically require the use of electrocatalysts to improve thereaction rate of the hydrogen oxidation reaction (HOR) and oxygenreduction reactions (ORR), which improves the PEMFC performance. PEMFCelectrocatalysts often comprise platinum, a relatively expensiveprecious metal. It is typically desirable to minimize the platinumcontent in PEMFC devices to minimize cost. Sufficient platinum content,however, is needed to provide sufficient catalytic activity and PEMFCdevice performance. As such, there is a desire to increase the catalystactivity per unit catalyst mass (mass activity). There are two generalapproaches to increase the mass activity, namely increasing the catalystactivity per unit catalyst surface area (specific activity) andincreasing the catalyst surface area per catalyst mass (specific surfacearea or specific area). The HOR and ORR occur on the catalyst surface,so increasing the specific surface area and/or the specific activity canreduce the amount of catalyst needed to achieve a desired absoluteperformance, reducing cost.

To maximize specific area, PEMFC electrocatalysts are often in the formof nanometer-scale thin films or particles on support materials. Anexemplary support material for nanoparticle PEMFC electrocatalysts iscarbon black, and an exemplary support material for thin filmelectrocatalysts is whiskers.

To increase the specific activity, PEMFC Pt ORR electrocatalysts oftenalso comprise certain transition metals such as cobalt or nickel.Without being bound by theory, incorporation of certain transitionmetals into the Pt lattice is believed to induce contraction of the Ptatoms at the catalyst surface, which increases the kinetic reaction rateby modification of the molecular oxygen binding and dissociationenergies and the binding energies of reaction intermediates and/orspectator species.

PEMFC electrocatalysts may incorporate other precious metals. Forexample, HOR PEMFC Pt electrocatalysts can be alloyed with ruthenium toimprove tolerance to carbon monoxide, a known Pt catalyst poison. HORand ORR PEMFC electrocatalysts may also incorporate iridium tofacilitate improved activity for the oxygen evolution reaction (OER).Improved OER activity may improve the durability of the PEMFC underinadvertent operation in the absence of fuel and during PEMFC systemstartup and shutdown. Incorporation of iridium into the PEMFC ORRelectrocatalyst, however, may result in decreased mass activity andhigher catalyst cost. Iridium has relatively lower specific activity forORR than platinum, potentially resulting in decreased mass activity.Iridium is also a precious metal, and thereby its incorporation canincrease cost. As such, the amount of iridium incorporated into PEMFCORR electrocatalysts should balance the improved OER activity anddecreased ORR activity.

PEMFC electrocatalysts may have different structural and compositionalmorphologies. The structural and compositional morphologies are oftentailored through specific processing methods during the electrocatalystfabrication, such as variations in the electrocatalyst deposition methodand annealing methods. PEMFC electrocatalysts can be compositionallyhomogenous, compositionally layered, or may contain compositiongradients throughout the electrocatalyst. Tailoring of compositionprofiles within the electrocatalyst may improve the activity anddurability of electrocatalysts. PEMFC electrocatalyst particles ornanometer-scale films may have substantially smooth surfaces or haveatomic- or nanometer-scale roughness. PEMFC electrocatalysts may bestructurally homogenous or may be nanoporous, being comprised ofnanometer-scale pores and solid catalyst ligaments.

As compared to structurally homogenous electrocatalysts, nanoporousPEMFC electrocatalysts may have higher specific area, thereby reducingcost. Nanoporous catalysts are comprised of numerous interconnectednanoscale catalyst ligaments, and the surface area of a nanoporousmaterial depends upon the diameter and volumetric number density of thenanoscale ligaments. Surface area is expected to increase as thenanoscale ligaments diameter decreases and the volumetric number densityincreases.

One method of forming nanoporous PEMFC electrocatalysts is viadealloying of a transition metal rich Pt alloy precursor, such as a PtNialloy with 30 at. % Pt and 70 at. % Ni. During dealloying, the precursoris exposed to conditions where the transition metal is dissolved and thesurface Pt has sufficient mobility to allow exposure of subsurfacetransition metal and formation of nanoscale ligaments which separate thenanopores. Dealloying to form nanopores can be induced via freecorrosion approaches, such as exposure to acid, or via exposure torepeated electrochemical oxidation and reduction cycles. Electrocatalystnanopore formation may occur spontaneously during electrochemicaloperation within a PEMFC, or may occur via ex-situ processing prior toPEMFC operation.

In PEMFC devices, electrocatalysts may lose performance over time, dueto a variety of degradation mechanisms which induce structural andcompositional changes. Such performance loss may shorten the practicallifetime of such systems. Electrocatalyst degradation may occur, forexample, due to loss of electrocatalyst activity per unit surface areaand loss of electrocatalyst surface area. Electrocatalyst specificactivity may be lost, for example, due to the dissolution ofelectrocatalyst alloying elements. Non-porous nanoparticle andnano-scale thin films may lose surface area, for example, due to Ptdissolution, particle sintering, and loss of surface roughness.Nanoporous electrocatalysts may additionally lose surface area, forexample, due to increased nanoscale ligament diameter and decreasednanoscale ligament density.

Additional electrocatalysts and systems containing such catalysts aredesired, including those that address one or more of the issuesdiscussed above.

SUMMARY

In one aspect, the present disclosure provides a nanoporous oxygenreduction catalyst material comprising PtNiIr. In some embodiments, thenanoporous oxygen reduction catalyst material has the formulaPt_(x)Ni_(y)Ir_(z), wherein x is in a range from 26.6 to 47.8, y is in arange from 48.7 to 70, and z is in a range from 1 to 11.4 (in someembodiments, x is in a range from 26.6 to 47.6, y is in a range from48.7 to 69.3, and z is in a range from 1 to 11.4; x is in a range from26.6 to 30, y is in a range from 17 to 62, and z is in a range from 1 to11.4; or even x is in a range from 47.6 to 47.8, y is in a range from48.7 to 52.2, and z is in a range from 0 to 3.7; or even in oneexemplary embodiment, x is 28.1, y is 64.9, and z is 7.0). In someembodiments, the catalyst material functions as an oxygen reductioncatalyst material.

In some embodiments, the nanoporous oxygen reduction catalyst materialhas pores with diameters in a range from 1 nm to 10 nm (in someembodiments, in a range from 2 nm to 8 nm, or even 3 nm to 7 nm).

In some embodiments, nanoporous oxygen reduction catalyst materialdescribed herein has been annealed.

Surprisingly, Applicants discovered the addition of iridium tonanoporous PtNi catalyst can substantially improve retention of massactivity, specific area, and/or performance after acceleratedelectrocatalyst aging. Iridium was observed to improve the durabilitywhether incorporated into the bulk of the catalysts or at the surface ofthe catalysts, whether incorporated into or at the surface of thecatalyst before or after annealing, and whether incorporated into or atthe surface of the catalyst before or after nanoporosity was formed viadealloying.

Nanoporous oxygen reduction catalyst materials described herein areuseful, for example, in fuel cell membrane electrode assemblies. Forexample, a catalyst used in a fuel cell membrane electrode assembly maycomprise nanostructured elements comprising microstructured supportwhiskers having an outer surface at least partially covered by thenanoporous oxygen reduction catalyst material described herein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of an exemplary catalyst described herein.

FIG. 1A is a side view of an exemplary catalyst described herein.

FIG. 2 is a schematic of an exemplary fuel cell.

FIG. 3A is a plot of the electrocatalyst mass activity of Examples 1-8and Comparative Example A catalysts, normalized to platinum content.

FIG. 3B is a plot of the electrocatalyst activity of Examples 1-8 andComparative Example A catalysts, normalized to total platinum groupmetal content.

FIG. 3C is a plot of the electrocatalyst surface area of Examples 1-8and Comparative Example A catalysts, normalized to platinum content.

FIG. 3D is a plot of the electrocatalyst surface area of Examples 1-8and Comparative Example A catalysts, normalized to total platinum groupmetal content.

FIG. 3E is a plot of fuel cell performance of Examples 1-8 andComparative Example A catalysts.

FIG. 4A is a plot of the change in electrocatalyst activity after adurability test, normalized to platinum content, for Examples 1-8 andComparative Example A.

FIG. 4B is a plot of the change in electrocatalyst surface area after adurability test, normalized to platinum content, for Examples 1-8 andComparative Example A.

FIG. 4C is a plot of the change in fuel cell performance after adurability test for Examples 1-8 and Comparative Example A.

FIG. 5A is a transmission electron micrograph (TEM) of ComparativeExample B at 225,000×.

FIG. 5B is a set of energy dispersive spectroscopy (EDS) elementalcomposition maps of Comparative Example B.

FIG. 6A is a transmission electron micrograph of Comparative Example Cat 225,000×.

FIG. 6B is a set of energy dispersive spectroscopy elemental compositionmaps of Comparative Example C.

FIG. 7A is a transmission electron micrograph of Example 9 at 225,000×.

FIG. 7B is a set of energy dispersive spectroscopy elemental compositionmaps of Example 9.

FIG. 8A is a linear composition profile of Comparative Example B, takenthrough the entire thickness of a catalyzed whisker, expressed aselement mass percent.

FIG. 8B is a linear composition profile of Comparative Example B, takenthrough the entire thickness of a catalyzed whisker, expressed aselement mole fraction.

FIG. 8C is a linear composition profile of Comparative Example C, takenthrough the entire thickness of a catalyzed whisker, expressed aselement mass percent.

FIG. 8D is a linear composition profile of Comparative Example C, takenthrough the entire thickness of a catalyzed whisker, expressed aselement mole fraction.

FIG. 8E is a linear composition profile of Example 9, taken through theentire thickness of a catalyzed whisker, expressed as element masspercent.

FIG. 8F is a linear composition profile of Example 9, taken through theentire thickness of a catalyzed whisker, expressed as element molefraction.

FIG. 9 is x-ray diffraction spectra of Comparative Examples B and C andExample 9.

FIG. 10A is a set of fuel cell performance curves of Comparative ExampleB, measured before and after a durability test.

FIG. 10B is a set of fuel cell performance curves of Comparative ExampleC, measured before and after a durability test.

FIG. 10C is a set of fuel cell performance curves of Example 9, measuredbefore and after a durability test.

FIG. 11A is a set of fuel cell performance curves of Comparative ExampleD, measured before and after a durability test.

FIG. 11B is a set of fuel cell performance curves of Example 10,measured before and after a durability test.

FIG. 11C is a set of fuel cell performance curves of Example 11,measured before and after a durability test.

DETAILED DESCRIPTION

Referring to FIG. 1, exemplary catalyst 100 on substrate 108 hasnanostructured elements 102 with microstructured whiskers 104 havingouter surface 105 at least partially covered by nanoporous oxygenreduction catalyst material 106 comprising PtNiIr.

One exemplary method for making nanoporous oxygen reduction catalystmaterial described herein comprises:

providing an oxygen reduction catalyst material comprising PtNiIr,wherein there are layers comprising platinum and nickel; and

dealloying at least some layers comprising platinum and nickel to removenickel from at least one layer to provide a nanoporous oxygen reductioncatalyst material described herein.

Another exemplary method for making nanoporous oxygen reduction catalystmaterial described herein comprises:

depositing platinum and nickel from a target comprising platinum andnickel to provide a first layer comprising platinum and nickel;

depositing a layer comprising iridium from a target comprising iridium;

repeating the preceding two steps, in order, at least once (in someembodiments, repeating 2, 3, 4, 5, 10, 15, 20, 25, 50, 75, 100, 150,200, 250, or even at least 275 times); and dealloying at least one layercomprising platinum and nickel to remove nickel from the layer toprovide a nanoporous oxygen reduction catalyst material describedherein.

Referring to FIG. 1A, in some embodiments, the layer(s) dealloyed ispart of a catalyst such as exemplary catalyst 1100 on substrate 1108having nanostructured elements 1102 with microstructured whiskers 1104having outer surface 1105 at least partially covered by catalystmaterial 1106 comprising PtNiIr which has pores 1110.

Suitable whiskers can be provided by techniques known in the art,including those described in U.S. Pat. No. 4,812,352 (Debe), U.S. Pat.No. 5,039,561 (Debe), U.S. Pat. No. 5,338,430 (Parsonage et al.), U.S.Pat. No. 6,136,412 (Spiewak et al.), and U.S. Pat. No. 7,419,741(Vernstrom et al.), the disclosures of which are incorporated herein byreference. In general, nanostructured whiskers can be provided, forexample, by vacuum depositing (e.g., by sublimation) a layer of organicor inorganic material onto a substrate (e.g., a microstructured catalysttransfer polymer sheet), and then, in the case of perylene reddeposition, converting the perylene red pigment into nanostructuredwhiskers by thermal annealing. Typically, the vacuum deposition stepsare carried out at total pressures at or below about 10-3 Torr or 0.1Pascal. Exemplary microstructures are made by thermal sublimation andvacuum annealing of the organic pigment C.I. Pigment Red 149 (i.e.,N,N′-di(3,5-xylyl)perylene-3,4:9,10-bis(dicarboximide)). Methods formaking organic nanostructured layers are reported, for example, inMaterials Science and Engineering, A158 (1992), pp. 1-6; J. Vac. Sci.Technol. A, 5, (4), July/August 1987, pp. 1914-16; J. Vac. Sci. Technol.A, 6, (3), May/August 1988, pp. 1907-11; Thin Solid Films, 186, 1990,pp. 327-47; J. Mat. Sci., 25, 1990, pp. 5257-68; Rapidly QuenchedMetals, Proc. of the Fifth Int. Conf. on Rapidly Quenched Metals,Wurzburg, Germany (Sep. 3-7, 1984), S. Steeb et al., eds., ElsevierScience Publishers B.V., New York, (1985), pp. 1117-24; Photo. Sci. andEng., 24, (4), July/August 1980, pp. 211-16; and U.S. Pat. No. 4,340,276(Maffitt et al.) and U.S. Pat. No. 4,568,598 (Bilkadi et al.), thedisclosures of which are incorporated herein by reference. Properties ofcatalyst layers using carbon nanotube arrays are reported in thearticle, “High Dispersion and Electrocatalytic Properties of Platinum onWell-Aligned Carbon Nanotube Arrays”, Carbon, 42, (2004), pp. 191-197.Properties of catalyst layers using grassy or bristled silicon arereported, for example, in U.S. Pat. App. Pub. No. 2004/0048466 A1 (Goreet al.).

Vacuum deposition may be carried out in any suitable apparatus (see,e.g., U.S. Pat. No. 5,338,430 (Parsonage et al.), U.S. Pat. No.5,879,827 (Debe et al.), U.S. Pat. No. 5,879,828 (Debe et al.), U.S.Pat. No. 6,040,077 (Debe et al.), and U.S. Pat. No. 6,319,293 (Debe etal.), and U.S. Pat. App. Pub. No. 2002/0004453 A1 (Haugen et al.), thedisclosures of which are incorporated herein by reference. One exemplaryapparatus is depicted schematically in FIG. 4A of U.S. Pat. No.5,338,430 (Parsonage et al.), and discussed in the accompanying text,wherein the substrate is mounted on a drum, which is then rotated over asublimation or evaporation source for depositing the organic precursor(e.g., perylene red pigment) prior to annealing the organic precursor inorder to form the whiskers.

Typically, the nominal thickness of deposited perylene red pigment is ina range from about 50 nm to 500 nm. Typically, the whiskers have anaverage cross-sectional dimension in a range from 20 nm to 60 nm and anaverage length in a range from 0.3 micrometer to 3 micrometers.

In some embodiments, the whiskers are attached to a backing. Exemplarybackings comprise polyimide, nylon, metal foils, or other material thatcan withstand the thermal annealing temperature up to 300° C. In someembodiments, the backing has an average thickness in a range from 25micrometers to 125 micrometers.

In some embodiments, the backing has a microstructure on at least one ofits surfaces. In some embodiments, the microstructure is comprised ofsubstantially uniformly shaped and sized features at least three (insome embodiments, at least four, five, ten, or more) times the averagesize of the whiskers. The shapes of the microstructures can, forexample, be V-shaped grooves and peaks (see, e.g., U.S. Pat. No.6,136,412 (Spiewak et al.), the disclosure of which is incorporatedherein by reference) or pyramids (see, e.g., U.S. Pat. No. 7,901,829(Debe et al.), the disclosure of which is incorporated herein byreference). In some embodiments, some fraction of the microstructurefeatures extend above the average or majority of the microstructuredpeaks in a periodic fashion, such as every 31st V-groove peak being 25%or 50% or even 100% taller than those on either side of it. In someembodiments, this fraction of features that extends above the majorityof the microstructured peaks can be up to 10% (in some embodiments up to3%, 2%, or even up to 1%). Use of the occasional taller microstructurefeatures may facilitate protecting the uniformly smaller microstructurepeaks when the coated substrate moves over the surfaces of rollers in aroll-to-roll coating operation. The occasional taller feature touchesthe surface of the roller rather than the peaks of the smallermicrostructures, so much less of the nanostructured material or whiskermaterial is likely to be scraped or otherwise disturbed as the substratemoves through the coating process. In some embodiments, themicrostructure features are substantially smaller than half thethickness of the membrane that the catalyst will be transferred to inmaking a membrane electrode assembly. This is so that during thecatalyst transfer process, the taller microstructure features do notpenetrate through the membrane where they may overlap the electrode onthe opposite side of the membrane. In some embodiments, the tallestmicrostructure features are less than ⅓rd or ¼th of the membranethickness. For the thinnest ion exchange membranes (e.g., about 10micrometers to 15 micrometers in thickness), it may be desirable to havea substrate with microstructured features no larger than about 3micrometers to 4.5 micrometers tall. The steepness of the sides of theV-shaped or other microstructured features or the included anglesbetween adjacent features may, in some embodiments, be desirable to beon the order of 900 for ease in catalyst transfer during alamination-transfer process and to have a gain in surface area of theelectrode that comes from the square root of two (1.414) surface area ofthe microstructured layer relative to the planar geometric surface ofthe substrate backing.

In some embodiments, the catalyst material to be dealloyed comprises alayer comprising platinum and nickel and a layer comprising iridium onthe layer comprising platinum and nickel.

In some embodiments, layers of catalyst material to be dealloyedcomprising platinum and nickel have a planar equivalent thickness in arange from 0.4 nm to 70 nm (in some embodiments, in a range from 0.4 nmto 10 nm, 0.4 nm to 5 nm, 1 nm to 25 nm, or even 1 nm to 10 nm) andlayers comprising iridium have a planar equivalent thickness (i.e., thethickness, if deposited on a substantially flat, planar substrate) in arange from 0.01 nm to 20 nm (in some embodiments, in a range from 0.01nm to 10 nm, 0.01 nm to 5 nm, 0.02 nm to 2.5 nm, or even 0.02 nm to 1nm). In some embodiments, layer(s) of catalyst material to be dealloyedcomprising platinum and nickel collectively has a planar equivalentthickness up to 600 nm (in some embodiments, up to 575 nm, 550 nm, 500nm, 400 nm, 300 nm, 200 nm, 100 nm, 75 nm, 50 nm, 25 nm, 10 nm, 5 nm,2.5 nm, 1 nm, or even up to two monolayers (e.g., 0.4 nm); in someembodiments, in a range from 0.4 nm to 600 nm, 0.4 nm to 500 nm, 1 nm to500 nm, 5 nm to 500 nm, 10 nm to 500 nm, 10 nm to 400 nm, or even 40 nmto 300 nm) and the layer comprising iridium has a planar equivalentthickness up to 50 nm (in some embodiments, up to 45 nm, 40 nm, 35 nm,30 nm, 25 nm, 20 nm, 15 nm, 10 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1 nm, amonolayer (e.g., 0.2 nm) or even less than a monolayer (e.g., 0.01 nm);in some embodiments, in a range from 0.01 nm to 50 nm, 1 nm to 50 nm, 5nm to 40 nm, or even 5 nm to 35 nm).

In some embodiments, catalyst material to be dealloyed comprisesalternating layers comprising platinum and nickel and layers comprisingiridium (i.e., a layer comprising platinum and nickel, a layercomprising iridium, a layer comprising platinum and nickel, a layercomprising iridium, etc.). In some embodiments, at least 2, 3, 4, 5, 10,15, 20, 25, 50, 75, 100, 150, 200, 250, or even at least 275 sets of thealternating layers.

The thickness of an individual deposited catalyst layer may depend, forexample, on the areal catalyst loading of the layer and the catalystdensity. For example, the thickness of a single layer of Pt with 10micrograms of Pt per cm² planar area and density of 21.45 g/cm³deposited onto a planar substrate is calculated as 4.7 nm, and thethickness of a Ni layer with the same areal loading is 11.2 nm.

In some embodiments, catalyst material to be dealloyed comprises a layercomprising platinum, a layer comprising nickel on the layer comprisingplatinum, and a layer comprising iridium on the layer comprising nickel.In some embodiments, catalyst material to be dealloyed comprises a layercomprising nickel, a layer comprising platinum on the layer comprisingnickel, and a layer comprising iridium on the layer comprising platinum.In some embodiments, catalyst material to be dealloyed comprisesrepeating sequential individual layers of platinum, nickel, and iridium.In some embodiments, at least 2, 3, 4, 5, 10, 15, 20, 25, 50, 75, 100,150, 200, 250, or even at least 275 sets of the repeating layers.

In some embodiments, catalyst material to be dealloyed has an exposediridium surface layer.

In some embodiments, each layer of catalyst material to be dealloyedcomprising platinum and nickel independently has a planar equivalentthickness up to 100 nm (in some embodiments, up to 50 nm, 20 nm, 15 nm,10 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1 nm, a monolayer (e.g., 0.2 nm), or evenup to less than a monolayer (e.g. 0.01 nm); in some embodiments, in arange from 0.01 nm to 100 nm, 0.01 nm to 50 nm, 0.1 nm to 15 nm, 0.1 nmto 10 nm, or even 1 nm to 5 nm).

In general, catalyst material to be dealloyed can be deposited bytechniques known in the art.

Exemplary deposition techniques include those independently selectedfrom the group consisting of sputtering (including reactive sputtering),atomic layer deposition, molecular organic chemical vapor deposition,molecular beam epitaxy, thermal physical vapor deposition, vacuumdeposition by electrospray ionization, and pulse laser deposition.Additional general details can be found, for example, in U.S. Pat. No.5,879,827 (Debe et al.), U.S. Pat. No. 6,040,077 (Debe et al.), and U.S.Pat. No. 7,419,741 (Vernstrom et al.), the disclosures of which areincorporated herein by reference. The thermal physical vapor depositionmethod uses suitable elevated temperature (e.g., via resistive heating,electron beam gun, or laser) to melt or sublimate the target (sourcematerial) into vapor state which is in turn passed through a vacuumspace, then condensing of the vaporized form onto substrate surfaces.Thermal physical vapor deposition equipment is known in the art,including that available, for example, as a metal evaporator or as anorganic molecular evaporator from CreaPhys GmbH, Dresden, Germany, underthe trade designations “METAL EVAPORATOR (ME-SERIES)” or “OrganicMolecular Evaporator (DE-SERIES)” respectively; another example of anorganic materials evaporator is available from Mantis Deposition LTD,Oxfordshire, UK, under the trade designation “ORGANIC MATERIALSEVAPORATIOR (ORMA-SERIES).” Catalyst material to be dealloyed comprisingmultiple alternating layers can be sputtered, for example, from multipletargets (e.g., Pt is sputtered from a first target, Ni is sputtered froma second target, and Ir from a third, or from a target(s) comprisingmore than one element (e.g., Pt and Ni)). If the catalyst coating isdone with a single target, it may be desirable that the coating layer beapplied in a single step onto the gas distribution layer, gas dispersionlayer, catalyst transfer layer, or membrane, so that the heat ofcondensation of the catalyst coating heats the underlying catalyst orsupport Pt, Ni, or Ir atoms as applicable, and substrate surface issufficient to provide enough surface mobility that the atoms are wellmixed and form thermodynamically stable alloy domains. Alternatively,for example, the substrate can also be provided hot or heated tofacilitate this atomic mobility. In some embodiments, sputtering isconducted, at least in part, in an atmosphere comprising argon.Organometallic forms of catalysts can be deposited, for example, by softor reactive landing of mass-selected ions. Soft landing of mass-selectedions is used to transfer catalytically-active metal complexes, completewith organic ligands, from the gas phase onto an inert surface. Thismethod can be used to prepare materials with defined active sites andthus achieve molecular design of surfaces in a highly-controlled wayunder either ambient or traditional vacuum conditions. For additionaldetails see, for example, Johnson et al., Anal. Chem., 2010, 82, pp.5718-5727, and Johnson et al., Chemistry: A European Journal, 2010, 16,pp. 14433-14438, the disclosures of which are incorporated herein byreference.

In some embodiments, the weight ratio of platinum to iridium of thecatalyst material before or after dealloying is in a range from 1:1 to50:1 (in some embodiments, in a range from 2:1 to 40:1).

In some embodiments, methods for making catalyst material that isdealloyed comprise depositing platinum and nickel from a targetcomprising platinum and nickel (e.g., a Pt₃Ni₇ target) and depositingiridium from a target comprising iridium. In some embodiments, layerscomprising platinum and nickel have a planar equivalent thickness in arange from 0.4 nm to 580 nm (in some embodiments, in a range from 0.4 nmto 72 nm) and layers comprising iridium have a planar equivalentthickness in a range from 0.01 nm to 32 nm (in some embodiments, in arange from 0.01 nm to 16 nm, or even a range from 0.01 nm to 2 nm).

In some embodiments, methods for making catalyst described hereincomprise depositing platinum from a target comprising platinum,depositing nickel from a target comprising nickel, and depositingiridium from a target comprising iridium. In some embodiments, a layercomprising platinum, an adjacent layer comprising nickel, and anadjacent layer comprising iridium collectively having a planarequivalent thickness in a range from 0.5 nm to 50 nm (in someembodiments, in a range from 0.5 nm to 30 nm). In some embodiments,layers comprising platinum have a planar equivalent thickness in a rangefrom 0.2 nm to 30 nm (in some embodiments, in a range from 0.2 nm to 20nm, or even 0.2 nm to 10 nm), layers comprising nickel have a planarequivalent thickness in a range from 0.2 nm to 50 nm (in someembodiments, in a range from 0.2 nm to 25 nm, or even 0.2 nm to 10 nm)and layers comprising iridium have a planar equivalent thickness in arange from 0.01 nm to 20 nm (in some embodiments, in a range from 0.01nm to 10 nm, 0.01 nm to 5 nm, 0.02 nm to 5 nm, 0.02 nm to 1 nm, or even0.1 nm to 1 nm). In some embodiments, the weight ratio of platinum toiridium is in a range from 2.4:1 to 34.3:1 (in some embodiments, in arange from 6.5:1 to 34.3:1, or even 9.7:1 to 34.3:1).

The nanoporosity is typically provided by dealloying the catalystmaterial to remove a portion of the nickel. In general, dealloying canbe accomplished by techniques known in the art, including via“free-corrosion” approaches (e.g., immersion in acid) or viaelectrochemical processing (e.g. potential cycling in acidic media).Nanoporosity formation typically occurs in alloys comprising at leasttwo components with sufficiently different dissolution rates in thedealloying medium and when the more noble component has sufficientsurface mobility. For additional details see, for example, Erlebacher etal., Nature, 2001, 410, pp. 450-453 and U.S. Pat. No. 6,805,972 B2(Erlebacher et al.); U.S. Pat. No. 8,673,773 B2 (Oppermann et al.); andU.S. Pat. No. 8,895,206 B2 (Erlebacher et al.), the disclosures of whichare incorporated herein by reference.

In some embodiments, catalyst material to be dealloyed or the(dealloyed) nanoporous oxygen reduction catalyst material is annealed.In some embodiments, the catalyst material is annealed beforedealloying. In general, annealing can be done by techniques known in theart, including heating the catalyst via, for example, an oven orfurnace, with a laser, and with infrared techniques. Annealing can beconducted, for example, in inert or reactive gas environments. Althoughnot wanting to be bound by theory, it is believed annealing can inducestructural changes on the atomic scale, which can influence activity anddurability of catalysts. Further, it is believed annealing nanoscaleparticles and films can induce mobility in the atomic constituent(s),which can cause growth of particles or thin film grains. In the case ofmulti-element mixtures, alloys, or layered particles and films, it isbelieved annealing can induce, for example, segregation of componentswithin the particle or film to the surface, formation of random,disordered alloys, and formation of ordered intermetallics, dependingupon the component element properties and the annealing environment. Foradditional details regarding annealing see, for example, van der Vlietet al., Nature Materials, 2012, 11, pp. 1051-1058; Wang et al., NatureMaterials, 2013, 12, pp. 81-87, and U.S. Pat. No. 8,748,330 B2 (Debe etal.), the disclosures of which are incorporated herein by reference.

In some embodiments, nanoporous oxygen reduction catalyst materialsdescribed herein are in the form of at least one nanoporous layercomprising platinum and nickel. In some embodiments, nanoporous layerscomprising platinum and nickel have a planar equivalent thicknesses(i.e., the thickness if deposited on a substantially flat, planarsubstrate) up to 600 nm (in some embodiments, up to 575 nm, 550 nm, 500nm, 400 nm, 300 nm, 200 nm, 100 nm, 75 nm, 50 nm, 25 nm, 10 nm, 5 nm,2.5 nm, 1 nm, or even up to two monolayers (e.g., 0.4 nm); in someembodiments, in a range from 0.4 nm to 600 nm, 0.4 nm to 500 nm, 1 nm to500 nm, 5 nm to 500 nm, 10 nm to 500 nm, 10 nm to 400 nm, or even 40 nmto 300 nm).

In some embodiments, nanoporous oxygen reduction catalyst materialdescribed herein, there is a layer comprising iridium on at least one ofthe nanoporous layers comprising platinum and nickel. In someembodiments, the catalyst material has an exposed iridium surface layer.In some embodiments, the layer comprising iridium has a planarequivalent thickness up to 50 nm (in some embodiments, up to 45 nm, 40nm, 35 nm, 30 nm, 25 nm, 20 nm, 15 nm, 10 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1nm, a monolayer (e.g., 0.2 nm) or even less than a monolayer (e.g., 0.01nm); in some embodiments, in a range from 0.01 nm to 50 nm, 1 nm to 50nm, 5 nm to 40 nm, or even 5 nm to 35 nm).

In some embodiments, nanoporous oxygen reduction catalyst materialdescribed herein has at least two crystalline phases present, whereinone of the crystalline phases is richer in Ni than the other crystallinephase. In some embodiments, there is a difference in at least onelattice parameter between the crystalline phase richer in Ni than theother crystalline phase is at least 3.5% (in some embodiments, in arange from 3.3% to 3.6%).

Catalysts described herein are useful, for example, in fuel cellmembrane electrode assemblies (MEAs). “Membrane electrode assembly”refers to a layered sandwich of fuel cell materials comprising amembrane, anode and cathode electrode layers, and gas diffusion layers.Typically, the cathode catalyst layer comprises a catalyst describedherein, although in some embodiments, the anode catalyst layerindependently comprises a catalyst described herein.

An MEA comprises, in order:

a first gas distribution layer having first and second opposed majorsurfaces;

an anode catalyst layer having first and second opposed major surfaces,the anode catalyst comprising a first catalyst;

an electrolyte membrane;

a cathode catalyst layer having first and second opposed major surfaces,the cathode catalyst comprising a second catalyst; and

a second gas distribution layer having first and second opposed majorsurfaces.

Electrolyte membranes conduct reaction intermediate ions between theanode and cathode catalyst layers. Electrolyte membranes preferably havehigh durability in the electrochemical environment, including chemicaland electrochemical oxidative stability. Electrolyte membranespreferably have low ionic resistance for the transport of the reactionintermediate ions, but are relatively impermeable barriers for otherions, electrons, and reactant species. In some embodiments, theelectrolyte membrane is a proton exchange membrane (PEM), which conductscations. In PEM fuel cells, the electrolyte membrane preferably conductsprotons. PEMs are typically a partially fluorinated or perfluorinatedpolymer comprised of a structural backbone and pendant cation exchangegroups. PEMs are available, for example, from E. I. du Pont de Nemoursand Company, Wilmington, Del., under the trade designation “NAFION;”Solvay, Brussels, Belgium, under the trade designation “AQUIVION;” 3MCompany, St. Paul, Minn., under the designation “3M PFSA MEMBRANE;” andAsahi Glass Co., Tokyo, Japan, under the trade designation “FLEMION.”

A gas distribution layer generally delivers gas evenly to the electrodesand, in some embodiments, conducts electricity. It also provides forremoval of water in either vapor or liquid form, in the case of a fuelcell. Gas distribution layers are typically porous to allow reactant andproduct transport between the electrodes and the flow field. Sources ofgas distribution layers include carbon fibers randomly oriented to formporous layers, in the form of non-woven paper or woven fabrics. Thenon-woven carbon papers are available, for example, from MitsubishiRayon Co., Ltd., Tokyo, Japan, under the trade designation “GRAFILU-105;” Toray Corp., Tokyo, Japan, under the trade designation “TORAY;”AvCarb Material Solutions, Lowell, Mass., under the trade designation“AVCARB;” SGL Group, the Carbon Company, Wiesbaden, Germany, under thetrade designation “SIGRACET;” Freudenberg FCCT SE & Co. KG, Fuel CellComponent Technologies, Weinheim, Germany, under the trade designation“FREUDENBERG;” and Engineered Fibers Technology (EFT), Shelton, Conn.,under the trade designation “SPECTRACARB GDL.” The woven carbon fabricsor cloths are available, for example, from Electro Chem Inc., Woburn,Mass., under the trade designations “EC-CC1-060” and “EC-AC-CLOTH;”NuVant Systems Inc., Crown Point, Ind., under the trade designations“ELAT-LT” and “ELAT;” BASF Fuel Cell GmbH, North America, under thetrade designation “E-TEK ELAT LT;” and Zoltek Corp., St. Louis, Mo.,under the trade designation “ZOLTEK CARBON CLOTH.” The non-woven paperor woven fabrics can be treated to modify its hydrophobicity (e.g.,treatment with a polytetrafluoroethylene (PTFE) suspension withsubsequent drying and annealing). Gas dispersion layers often comprise aporous layer of sub-micrometer electronically-conductive particles(e.g., carbon), and a binder (e.g., PTFE). Although not wanting to bebound by theory, it is believed that gas dispersion layers facilitatereactant and product water transport between the electrode and the gasdistribution layers.

At least one of the anode or cathode catalyst has whiskers withnanoporous oxygen reduction catalyst material described herein. The“other catalyst layer” can be a conventional catalyst known in the art,and provided by techniques known in the art (e.g., U.S. Pat. No.5,759,944 (Buchanan et al.), U.S. Pat. No. 5,068,161 (Keck et al.), andU.S. Pat. No. 4,447,506 (Luczak et al.)), the disclosures of which areincorporated herein by reference.

In some embodiments, the cathode and/or anode catalyst layer compriseswhiskers with nanoporous oxygen reduction catalyst material describedherein.

A fuel cell is an electrochemical device that combines hydrogen fuel andoxygen from the air to produce electricity, heat, and water. Fuel cellsdo not utilize combustion, and as such, fuel cells produce little, ifany, hazardous effluents. Fuel cells convert hydrogen fuel and oxygendirectly into electricity, and can be operated, for example, at muchhigher efficiencies than internal combustion electric generators.

Referring to FIG. 2, exemplary fuel cell 200 includes first gasdistribution layer 201 adjacent to anode 203. Adjacent anode 203 is anelectrolyte membrane 204. Cathode 205 is situated adjacent electrolytemembrane 204, and second gas distribution layer 207 is situated adjacentcathode 205. In operation, hydrogen fuel is introduced into the anodeportion of the fuel cell 200, passing through first gas distributionlayer 201 and over anode 203. At anode 203, the hydrogen fuel isseparated into hydrogen ions (H⁺) and electrons (e⁻).

Electrolyte membrane 204 permits only the hydrogen ions or protons topass through electrolyte membrane 204 to the cathode portion of fuelcell 200. The electrons cannot pass through electrolyte membrane 204and, instead, flow through an external electrical circuit in the form ofelectric current. This current can power an electric load 217, such asan electric motor, or be directed to an energy storage device, such as arechargeable battery.

Oxygen flows into the cathode side of fuel cell 200 via seconddistribution layer 207. As the oxygen passes over cathode 205, oxygen,protons, and electrons combine to produce water and heat.

EXEMPLARY EMBODIMENTS

1A. A nanoporous oxygen reduction catalyst material comprising PtNiIr.

2A. The nanoporous oxygen reduction catalyst material of ExemplaryEmbodiment 1A having pores with diameters in a range from 1 nm to 10 nm(in some embodiments, in a range from 2 nm to 8 nm, or even 3 nm to 7nm).

3A. The nanoporous oxygen reduction catalyst material of any preceding AExemplary Embodiment, wherein the PtNiIr material has the formulaPt_(x)Ni_(y)Ir_(z), and wherein x is in a range from 26.6 to 47.8, y isin a range from 48.7 to 70, and z is in a range from 1 to 11.4 (in someembodiments, x is in a range from 26.6 to 47.6, y is in a range from48.7 to 69.3, and z is in a range from 1 to 11.4; x is in a range from26.6 to 30, y is in a range from 17 to 62, and z is in a range from 1 to11.4; or even x is in a range from 47.6 to 47.8, y is in a range from48.7 to 52.2, and z is in a range from 0 to 3.7; or even in oneexemplary embodiment, x is 28.1, y is 64.9, and z is 7.0).4A. The nanoporous oxygen reduction catalyst material of any preceding AExemplary Embodiment, in the form of at least one nanoporous layercomprising platinum and nickel.5A. The nanoporous oxygen reduction catalyst of Exemplary Embodiment 4A,wherein the nanoporous layers comprising platinum and nickel have aplanar equivalent thicknesses up to 600 nm (in some embodiments, up to575 nm, 550 nm, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 75 nm, 50 nm, 25nm, 10 nm, 5 nm, 2.5 nm, 1 nm, or even up to two monolayers (e.g., 0.4nm); in some embodiments, in a range from 0.4 nm to 600 nm, 0.4 nm to500 nm, 1 nm to 500 nm, 5 nm to 500 nm, 10 nm to 500 nm, 10 nm to 400nm, or even 40 nm to 300 nm).6A. The nanoporous oxygen reduction catalyst material of any ofExemplary Embodiments 4A or 5A, wherein a there is a layer comprisingiridium on at least one of the nanoporous layers comprising platinum andnickel.7A. The nanoporous oxygen reduction catalyst material of ExemplaryEmbodiment 6A, wherein the layer comprising iridium has a planarequivalent thickness up to 50 nm (in some embodiments, up to 45 nm, 40nm, 35 nm, 30 nm, 25 nm, 20 nm, 15 nm, 10 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1nm, a monolayer (e.g., 0.2 nm) or even less than a monolayer (e.g., 0.01nm); in some embodiments, in a range from 0.01 nm to 50 nm, 1 nm to 50nm, 5 nm to 40 nm, or even 5 nm to 35 nm).8A. The nanoporous oxygen reduction catalyst material of any preceding AExemplary Embodiment having an exposed iridium surface layer.9A. The nanoporous oxygen reduction catalyst material of any preceding AExemplary Embodiment, wherein there are at least two crystalline phasespresent, wherein one of the crystalline phases is richer in Ni than theother crystalline phase. In some embodiments, there is a difference inat least one lattice parameter between the crystalline phase richer inNi than the other crystalline phase by at least 3.5% (in someembodiments, in a range from 3.3% to 3.6%).10A. The nanoporous oxygen reduction catalyst material of any precedingA Exemplary Embodiment, wherein the weight ratio of platinum to iridiumis in a range from 1:1 to 50:1 (in some embodiments, in a range from 2:1to 40:1).11A. A catalyst comprising nanostructured elements comprisingmicrostructured support whiskers having an outer surface at leastpartially covered by the nanoporous oxygen reduction catalyst materialof any preceding A Exemplary Embodiment.12A. A fuel cell membrane electrode assembly comprising the catalyst ofExemplary Embodiment 11A.1B. A method comprising:

providing an oxygen reduction catalyst material comprising PtNiIr,wherein there are layers comprising platinum and nickel; and

dealloying at least some layers comprising platinum and nickel to removenickel from at least one layer to provide the nanoporous oxygenreduction catalyst material of any of Exemplary Embodiments 1A to 10A.In some embodiments, there are pores with diameters in a range from 1 nmto 10 nm (in some embodiments, in a range from 2 nm to 8 nm, or even 3nm to 7 nm) where the nickel was removed.

2B. The method of Exemplary Embodiment 1B, further comprising annealingthe catalyst before dealloying.

3B. The method of any preceding B Exemplary Embodiment, furthercomprising depositing platinum and nickel from a target comprisingplatinum and nickel and depositing iridium from a target comprisingiridium.

4B. The method of Exemplary Embodiment 3B, wherein the target is aPt₃Ni₇ target.

5B. A method of making the catalyst of either Exemplary Embodiment 1B or2B, further comprising depositing platinum from a target comprisingplatinum, depositing nickel from a target comprising nickel, anddepositing iridium from a target comprising iridium.6B. The method of any preceding B Exemplary Embodiment, wherein layersof the oxygen reduction catalyst material, before dealloying, comprisingplatinum and nickel having a planar equivalent thickness in a range from0.4 nm to 580 nm (in some embodiments, in a range from 0.4 nm to 72 nm)and layers comprising iridium have a planar equivalent thickness in arange from 0.01 nm to 32 nm (in some embodiments, in a range from 0.01nm to 16 nm, or even a range from 0.01 nm to 2 nm).7B. The method of any preceding B Exemplary Embodiment, wherein theweight ratio of platinum to iridium is in a range from 2.4:1 to 34.3:1(in some embodiments, in a range from 6.5:1 to 34.3:1, or even 9.7:1 to34.3:1).1C. A method of making the catalyst of any of Exemplary Embodiment 1A to10A, the method comprising:

depositing platinum and nickel from a target comprising platinum andnickel to provide a first layer comprising platinum and nickel;

depositing a layer comprising iridium from a target comprising iridium;

repeating the preceding two steps, in order, at least once (in someembodiments, repeating 2, 3, 4, 5, 10, 15, 20, 25, 50, 75, 100, 150,200, 250, or even at least 275 times); and

dealloying at least one layer comprising platinum and nickel to removenickel from the layer. In some embodiments, there are pores withdiameters in a range from 1 nm to 10 nm (in some embodiments, in a rangefrom 2 nm to 8 nm, or even 3 nm to 7 nm) where the nickel was removed.

2C. The method of Exemplary Embodiment 1C, wherein the target is aPt₃Ni₇ target.

3C. The method of any preceding C Exemplary Embodiment, furthercomprising annealing the layers before dealloying.

4C. The method of any preceding C Exemplary Embodiment, wherein layerscomprising platinum and nickel have a planar equivalent thickness in arange from 0.4 nm to 70 nm (in some embodiments, in a range from 0.4 nmto 10 nm, 0.4 nm to 5 nm, 1 nm to 25 nm, or even 1 nm to 10 nm) andlayers comprising iridium have a planar equivalent thickness in a rangefrom 0.01 nm to 20 nm (in some embodiments, in a range from 0.01 nm to10 nm, 0.01 nm to 5 nm, 0.02 nm to 2.5 nm, or even 0.02 nm to 1 nm).

Advantages and embodiments of this invention are further illustrated bythe following examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. All parts andpercentages are by weight unless otherwise indicated.

EXAMPLES Examples 1-4

Nanostructured whiskers employed as catalyst supports were madeaccording to the process described in U.S. Pat. No. 5,338,430 (Parsonageet al.), U.S. Pat. No. 4,812,352 (Debe), and U.S. Pat. No. 5,039,561(Debe), incorporated herein by reference, using as substrates themicrostructured catalyst transfer substrates (or MCTS) described in U.S.Pat. No. 6,136,412, also incorporated herein by reference. Perylene redpigment (i.e., N,N′-di(3,5-xylyl)perylene-3,4:9,10-bis(dicarboximide))(C.I. Pigment Red 149, also known as “PR149”, obtained from Clariant,Charlotte, N.C.) was sublimation vacuum coated onto MCTS with a nominalthickness of 200 nm, after which it was annealed. After deposition andannealing, highly oriented crystal structures were formed with largeaspect ratios, controllable lengths of about 0.5 to 2 micrometers,widths of about 0.03-0.05 micrometer and areal number density of about30 whiskers per square micrometer, oriented substantially normal to theunderlying substrate.

Nanostructured thin film (NSTF) catalyst layers were prepared by sputtercoating catalyst films sequentially using a DC-magnetron sputteringprocess onto the layer of nanostructured whiskers. A vacuum sputterdeposition system (obtained as Model Custom Research from Mill LaneEngineering Co., Lowell, Mass.) equipped with 4 cryo-pumps (obtainedfrom Austin Scientific, Oxford Instruments, Austin, Tex.), a turbopumpand using typical Ar sputter gas pressures of about 5 mTorr (0.66 Pa),and 2 inch×10 inch (5 cm×25.4 cm) rectangular sputter targets (obtainedfrom Sophisticated Alloys, Inc., Butler, Pa.) was used. The coatingswere deposited by using ultra high purity Ar as the sputtering gas. Ptand Ni were first simultaneously deposited from a single alloy Pt₃Ni₇target (30 at. % Pt and 70 at. % Ni, obtained from Sophisticated Alloys,Butler, Pa.). 50 layers of Pt₃Ni₇ were deposited, each with about 2.8 nmplanar equivalent thickness, resulting in an areal Pt loading of about0.10 mg_(Pt)/cm². Pt₃Ni₇ catalysts deposited from a single alloy targetare referred to as “single target” (ST). Ir (obtained from Materion,Mayfield Heights, Ohio) was then subsequently deposited onto the surfaceof four pieces of the Pt₃Ni₇-coated NSTF catalyst on substrate, eachwith a different Ir areal loading calculated to yield 1, 2, 5, and 10at. % Ir content in the electrocatalyst (Examples 1, 2, 3, and 4,respectively). The Ir layer planar equivalent thickness for Examples 1,2, 3, and 4 was 1.5 nm, 2.9 nm, 7.1 nm, and 12.7 nm, respectively.

Representative areas of the electrocatalysts were analyzed for bulkcomposition using X-Ray Fluorescence spectroscopy (XRF). Representativecatalyst samples were evaluated on MCTS using a wavelength dispersiveX-ray fluorescence spectrometer (obtained under the trade designation“PRIMUS II” from Rigaku Corporation, Tokyo, Japan) equipped with arhodium (Rh) X-ray source, a vacuum atmosphere, and a 20-mm diametermeasurement area. Each sample was analyzed three times to obtain theaverage and standard deviation for the measured Pt, Ni, and Ir signalintensities, which are proportional to loading. The areal loading forthe electrocatalysts Pt, Ni, and Ir of Examples 1-4 were determined bycomparing their measured XRF intensities to the XRF intensities obtainedwith standard NSTF electrocatalysts containing Pt, Ni, and Ir with knownareal loadings. From the XRF-determined Pt, Ni, and Ir areal loadings,the catalysts' composition and Pt-to-Ir weight ratios were calculated.The total platinum group metal (PGM) content was determined by addingthe Pt and Ir areal loadings. Loading and composition information isprovided in Table 1, below.

TABLE 1 Pt:Ir PtNi Ir Loading, mg/cm² Composition, at. % Weight ExampleDeposition Incorporation Pt Ni Ir PGM Pt Ni Ir Ratio Comp. ST None 0.1040.073 0.000 0.104 30.0 70.0 0.0 Infinite Ex. A Ex. 1 ST Top layer 0.1030.072 0.003 0.106 29.7 69.3 1.0 31.1 Ex. 2 ST Top layer 0.103 0.0720.007 0.109 29.4 68.7 1.9 15.7 Ex. 3 ST Top layer 0.104 0.073 0.0160.120 28.7 66.9 4.4 6.6 Ex. 4 ST Top layer 0.102 0.072 0.029 0.131 27.664.5 7.9 3.6 Ex. 5 ST Bilayer 0.102 0.072 0.003 0.105 29.7 69.3 1.0 31.7Ex. 6 ST Bilayer 0.101 0.071 0.007 0.108 29.4 68.6 2.1 14.5 Ex. 7 STBilayer 0.107 0.075 0.011 0.118 29.1 67.9 3.0 9.8 Ex. 8 ST Bilayer 0.1000.070 0.042 0.142 26.6 62.0 11.4 2.4

The Pt_(x)Ni_(y)Ir_(z) catalysts and NSTF PtCoMn coated anode catalystwhiskers (0.05 mg_(Pt)/cm², Pt₆₉Co₂₈Mn₃) on MCTS were then transferredto either side of a 24-micrometer thick proton exchange membrane(available under the trade designation “3M PFSA 825EW” (neat) from 3MCompany, St. Paul, Minn.), using a laminator (obtained under the tradedesignation “HL-101” from ChemInstruments, Inc., West Chester Township,Ohio) to form a catalyst coated membrane (CCM). The three-layer stack-upwas hand fed into the laminator with hot nip rolls at 270° F. (132° C.),150 psi (1.03 MPa) nip, and rotating at the equivalent of 0.5 fpm (0.25cm/s). Immediately after lamination, the MCTS layers were peeled back,leaving the catalyst coated whiskers embedded into either side of thePEM. The CCM was installed with identical gas diffusion layers(available under the trade designation “3M 2979 GAS DIFFUSION LAYERS”from 3M Company) on the anode and cathode in 50 cm² active area testcells (obtained under the trade designation “50 CM2 CELL HARDWARE” fromFuel Cell Technologies, Inc., Albuquerque, N. Mex.) with quad-serpentineflow fields with gaskets selected to give 10% compression of the gasdiffusion layers. The catalyst of the present invention was evaluated asthe fuel cell cathode.

After assembly, the test cells were connected to a test station(obtained under the trade designation “SINGLE FUEL CELL TEST STATION”from Fuel Cell Technologies, Inc.). The MEA was then operated for about40 hours under a conditioning protocol to achieve apparent steady stateperformance. The protocol consisted of repeated cycles of operationaland shutdown phases, each about 40 and 45 minutes in duration,respectively. In the operational phase, the MEA was operated at 75° C.cell temperature, 70° C. dew point, 101/101 kPaA H₂/Air, with constantflow rates of 800 and 1800 sccm of H₂ and air, respectively. During the40-minute operational phase, the cell voltage was alternated between5-minute long polarization cycles between 0.85 V and 0.25 V and 5-minutelong potential holds at 0.40 V. During the 45-minute shutdown phase, thecell potential was set to open circuit voltage, H₂ and air flows to thecell were halted, and the cell temperature was cooled towards roomtemperature while liquid water was injected into the anode and cathodecell inlets at 0.26 g/min. and 0.40 g/min., respectively. Without beingbound by theory, it is believed the fuel cell conditioning protocol,which includes numerous potential cycles, may induce formation ofnanopores within the electrocatalyst.

After conditioning the MEA, the electrocatalysts were characterized forrelevant beginning of life (BOL) characteristics, including catalystactivity, surface area, and operational performance under relevantH₂/Air test conditions, described as follows.

The cathode oxygen reduction reaction (ORR) absolute activity wasmeasured with saturated 150 kPaA H₂/O₂, 80° C. cell temperature for 1200seconds at 900 mV vs. the 100% H₂ reference/counter electrode. The ORRabsolute activity (A/cm² or mA/cm²) was obtained by adding the measuredcurrent density after 1050 seconds of hold time and the electronicshorting and hydrogen crossover current densities, estimated from 2 mV/scyclic voltammograms measured with N₂ fed to the working electrodeinstead of O₂. The electrocatalyst mass activity, a measure of thecatalyst activity per unit precious metal content, is calculated bydividing the corrected ORR absolute activity (A/cm² _(planar)) by thecathode Pt or PGM areal loading (mg/cm²) to obtain the mass activity(A/mg_(Pt) or A/mg_(PGM)). The electrocatalyst mass activity is providedin Table 2, below, and FIGS. 3A and 3B.

TABLE 2 Ir H₂/Air Ir Content Specific Area Mass Activity PerformanceExample Incorporation at. % m²/g_(Pt) m²/g_(PGM) A/mg_(Pt) A/mg_(PGM)Volts Comp. none 0 16.1 16.1 0.35 0.35 0.892 Ex. A Ex. 1 Top layer 1.016.1 15.6 0.34 0.33 0.895 Ex. 2 Top layer 2.0 17.4 16.2 0.35 0.32 0.896Ex. 3 Top layer 3.0 18.8 17.1 0.31 0.28 0.897 Ex. 4 Top layer 11.4 19.013.3 0.15 0.11 0.879 Ex. 5 Bilayer 1.0 15.5 15.1 0.28 0.27 0.898 Ex. 6Bilayer 1.9 17.0 16.0 0.37 0.35 0.896 Ex. 7 Bilayer 4.4 19.2 16.6 0.340.30 0.896 Ex. 8 Bilayer 7.9 20.7 16.2 0.30 0.24 0.896

The cathode catalyst surface enhancement factor (SEF, m² _(Pt)/m²_(planar) or analogously cm² _(Pt)/cm² _(planar)) was measured viacyclic voltammetry (100 mV/s, 0.65 V-0.85 V, average of 100 scans) undersaturated 101 kilopascals absolute pressure (kPaA) H₂/N₂ and 70° C. celltemperature. The SEF was estimated by taking the average of theintegrated hydrogen underpotential deposition (H_(UPD)) charge (C/cm²_(planar)) for the oxidative and reductive waves and dividing by 220μC/cm²Pt. The electrocatalyst's specific surface area (m² _(Pt)/g_(pt)or m² _(Pt)/g_(PGM)), a measure of catalyst dispersion, was calculatedby dividing the SEF (m² _(Pt)/m² _(planar)) by the areal Pt or totalplatinum group metal (PGM) loading (g_(Pt)/m² _(planar) or g_(PGM)/m²_(planar)). The electrocatalyst specific area is provided in Table 2,above, and FIGS. 3C and 3D.

The operational performance of electrocatalysts was evaluated via H₂/Airpolarization curves, measured at 80° C. cell temperature, 68° C. dewpoint, 150/150 kPaA H₂/Air, with constant stoichiometry of 2.0 H₂ and2.5 for air. The current density was initially set to 20 mA/cm², andthen stepwise increased while the cell voltage was maintained above 0.40V, after which the current density was stepwise decreased back to 20mA/cm². The cell was held at each current density for 2 minutes. Thecell voltage at a specific current density, 20 mA/cm², is reported as“H₂/Air Performance” and is reported in Table 2, above, and FIG. 3E.

Example 1, 2, and 3 catalysts were additionally evaluated under anaccelerated stress test (AST), which evaluated the stability of theelectrocatalyst metal. In this test, the cell was operated at 80° C.cell temperature, 200/200 sccm H₂/N₂, 101 kPaA, 100% inlet relativehumidity (RH), and the cathode electrode potential was cycled between0.6 V-1.0 V vs. the hydrogen counter/reference electrode at a scan rateof 50 mV/s. Without being bound by theory, it is believed the ASTprotocol, which includes numerous potential cycles, may induce formationof nanopores within the electrocatalyst. After 10 or 15 thousand ASTcycles, the MEA was reconditioned for about 16 hours using the initialconditioning protocol, after which the cathode surface area, ORRactivity, and H₂/Air polarization curves were again measured todetermine the rate and extent of performance loss. This process of AST,reconditioning, and characterization was repeated such that the cell wasexposed to a total of 30,000 AST cycles. The changes in specific area,mass activity, and H₂/Air performance after the 30,000 AST cycles arelisted in Table 3, below, and shown in FIGS. 4A, 4B, and 4C.

TABLE 3 Pt Mass H₂/Air Ir Specific Activity Performance # of Samples IrContent Area Change Change Change Example Evaluated Incorporation at. %% m²/g_(Pt) % A/mg_(Pt) Volts Comp. Ex. A 1 none 0.0 −39.5 −61.6 −0.037Ex. 1 2 Top layer 1.0 −28.1 −40.3 −0.025 Ex. 2 2 Top layer 1.9 −25.4−35.1 −0.018 Ex. 3 1 Top layer 4.4 −24.7 −40.3 −0.022 Ex. 7 1 Bilayer3.0 −24.0 −37.5 −0.023

Examples 5-8

Examples 5-8 were prepared and evaluated as described for Examples 1-4,except that the Ir metal was incorporated into the alloy duringdeposition of the Pt₃Ni₇, and only Example 7 was evaluated fordurability with the AST protocol.

Four electrocatalysts were generated with varying Ir content. For each,a first “ST” Pt₃Ni₇ layer was deposited with about 1 nm planarequivalent thickness, onto which an Ir layer was deposited. The Irplanar equivalent thicknesses were about 0.01 nm, 0.02 nm, 0.04 nm, and0.14 nm for Examples 5, 6, 7, and 8, respectively. This depositionprocess was repeated 135 times until an areal Pt loading of about 0.10mg_(Pt)/cm² was achieved.

Loading and composition information is provided in Table 1, above. Thecatalyst mass activity, specific area, and H₂/Air performance afterinitial conditioning are reported in Table 2, above, and shown in FIGS.3A, 3B, 3C, 3D, and 3E. The changes in specific area, mass activity, andH₂/Air performance of Example 7 after the 30,000 AST cycles tested arelisted in Table 3, above, and shown in FIGS. 4A, 4B, and 4C.

Comparative Example A

Comparative Example A was prepared and evaluated as described forExample 1, except that no Ir was incorporated into the catalyst.

Loading and composition information is provided in Table 1, above. Thecatalyst specific area, mass activity, and H₂/Air performance afterinitial conditioning are reported in Table 2, above, and shown in FIGS.3A, 3B, 3C, 3D, and 3E. The changes in specific area, mass activity, andH₂/Air performance after the 30,000 AST cycles tested are listed inTable 3, above, and shown in FIGS. 4A, and 4C.

Comparative Example B

Comparative Example B was prepared as generally described forComparative Example A, except that during electrocatalyst deposition thePt₃Ni₇ loading and layer planar equivalent thickness differed. Threelayers of Pt₃Ni₇ were deposited, each with about 57 nm planar equivalentthickness, resulting in a Pt areal loading of about 0.13 mg_(Pt)/cm².Loading and composition information is provided in Table 4, below.

TABLE 4 Pt:Ir PtNi Loading, mg/cm² Composition, at. % Weight ExampleDeposition Process Pt Ni Ir PGM Pt Ni Ir Ratio Comp. Ex. B ST none 0.1310.097 0.000 0.131 28.9 71.1 0.0 Infinite Comp. Ex. C ST Dealloyed 0.1340.044 0 0.134 47.8 52.2 0.0 Infinite Ex. 9 ST Dealloyed, 0.133 0.0410.010 0.143 47.6 48.7 3.7 13.3 Ir top layer

Comparative Example B was analyzed as described for Comparative ExampleA, but additional composition and structural analysis was alsoperformed.

Two representative sections of catalyst were analyzed for bulkcrystalline structure using x-ray diffraction (XRD). Electrocatalysts onMCTS were analyzed via reflection geometry and data were collected inthe form of a survey scan by use of a vertical diffractometer(PANalytical, Almelo, The Netherlands), copper K, radiation, and PIXceldetector registry of the scattered radiation. The diffractometer wasfitted with variable incident beam slits and fixed diffracted beamslits. The survey scan was conducted from 30 to 55 degrees (2θ) using a0.05-degree step size and 5500-second dwell time setting. X-raygenerator settings of 40 kV and 40 mA were employed. A representativeXRD spectra for Comparative Example B is shown in FIG. 9. Table 5,below, provides (111) grain sizes and lattice constants for the phase(s)detected from the XRD spectra taken from two representative catalystsections.

TABLE 5 FCC Phase 1 FCC Phase 1 FCC Phase 2 FCC Phase 2 Number of (111)Apparent (111) Lattice (111) Apparent (111) Lattice Example FCC PhasesCrystallite Size, Å Parameter, Å Crystallite Size, Å Parameter, Å Comp.Ex. B 1 106 3.694 — — Comp. Ex. B 1 108 3.692 — — Comp. Ex. C 2 1493.689 43 3.766 Comp. Ex. C 2 145 3.692 44 3.787 Comp. Ex. C 2 146 3.69745 3.78 Comp. Ex. C 2 152 3.691 44 3.769 Ex. 9 2 144 3.687 44 3.819 Ex.9 2 146 3.689 45 3.814 Ex. 9 2 145 3.688 44 3.815 Ex. 9 2 148 3.688 413.815

A representative section of catalyst was evaluated for nanometer-scalestructure and composition using transmission electron microscopy (TEM)(obtained under the trade designation “OSIRIS” from FEI, Hillsboro,Oreg.) and energy dispersive spectroscopy (EDS) (obtained under thetrade designation “QUAD X-RAY DETECTOR” from Bruker, Billerica, Mass.),and with associated software (obtained under the trade designation“ESPRIT 1.9” from Bruker). For TEM and EDS analysis, the whiskers werescraped from the MCTS with a freshly broken bamboo splint to detach somewhiskers and transfer them to a TEM grid coated with a thin carbon film.The samples were imaged in the TEM at 200 kV accelerating voltage. Atleast two different sample grid areas were viewed. The raw elementalmaps were quantified to account for beam-spreading, absorption andfluorescence to produce quantitative weight percentage based elementalmaps. A dark field TEM image of Comparative Example B is shown in FIG.5A. A set of EDS element maps for C, Pt, Ni, and Ir, taken of the samearea as shown in FIG. 5A, showing the spatial distribution of elementsis shown in FIG. 5B. A linear composition profile of Comparative ExampleB, taken through the entire thickness of a catalyzed whisker detected byEDS, is plotted as element weight percentage in FIG. 8A. A linearcomposition profile through the entire thickness of a catalyzed whisker,calculated as Pt, Ni, and Ir mole fractions from the Pt, Ni, and Irweight percentage data shown in FIG. 8A, is shown in FIG. 8B.

After the additional compositional and structural analysis, the catalystwas assembled into fuel cells and evaluated for BOL performance and ASTdurability as described for Example 1. Catalyst performance anddurability metrics evaluated in fuel cell testing are provided in Table6, below. H₂/Air polarization curves, taken before the AST (“BOT”) andafter the AST (“AST”), are shown in FIG. 10A.

TABLE 6 H₂/Air Ir Performance Content Specific Area ORR Mass Activity @0.02 A/cm² Example Process at. % m²/g_(PGM) % Change A/mg_(PGM) % ChangeVolts Change Comp. Ex. B None 0 14.3 −39.6 0.297 −54.2 0.903 −0.037Comp. Ex. C Dealloyed 0 13.3 −39.7 0.287 −63.6 0.902 −0.042 Ex. 9Dealloyed, 3.7 12.9 −32.4 0.215 −42.4 0.896 −0.022 Ir top layer

Comparative Example C

Comparative Example C was generally prepared and analyzed as describedfor Comparative Example B, except that the Pt₃Ni₇ catalyst was dealloyedafter deposition. The electrocatalyst on MCTS was placed in contact witha gold-plated mesh electrode and installed into a custom laboratoryelectrochemical dealloying cell. The cell's counter electrode consistedof platinized titanium. The cell's reference electrode was an Hg/HgSO₄electrode. Aqueous sulfuric acid solution (1 M, RT, aq.) was theelectrolyte. The catalyst's potential was cycled several times between 0and 1.2 V-1.4 V vs. standard hydrogen electrode with a scan rate of 50mV/sec. Composition analysis by XRF was conducted after dealloying.

Loading and composition information is provided in Table 4, above.Catalyst performance and durability metrics evaluated in fuel celltesting are provided in Table 6, above. Four representative sectionswere analyzed by XRD. A representative XRD spectra for ComparativeExample C is shown in FIG. 9. Table 5 provides (111) grain sizes andlattice constants for the phase(s) detected in the four representativecatalyst sections analyzed by XRD. A dark field TEM image of ComparativeExample C is shown in FIG. 6A; arrows denote regions wherenanometer-scale pores are evident. A set of EDS element maps for C, Pt,Ni, and Ir, taken of the same area as FIG. 6A, showing the spatialdistribution of elements is shown in FIG. 6B. A linear compositionprofile through the entire thickness of a catalyzed whisker detected byEDS, plotted as element weight percentage, is shown in FIG. 8C. A linearcomposition profile through the entire thickness of a catalyzed whisker,calculated as Pt, Ni, and Ir mole fractions from the Pt, Ni, and Irweight percentage data shown in FIG. 8C, is shown in FIG. 8D. H₂/Airpolarization curves, taken before the AST (“BOT”) and after the AST(“AST”), are shown in FIG. 10B.

Example 9

Example 9 was generally prepared and analyzed as described forComparative Example C, except that Ir was deposited onto the surfaceafter dealloying. The dealloyed electrocatalyst was reloaded into thesputter deposition system and a single layer of Ir was deposited ontothe surface, with an areal loading of 0.01 mg_(Ir)/cm².

Loading and composition information is provided in Table 4, above.Catalyst performance and durability metrics evaluated in fuel celltesting are provided in Table 6, above. Four representative sectionswere analyzed by XRD. A representative XRD spectra for Example 9 isshown in FIG. 9. Table 5, above, provides (111) grain sizes and latticeconstants for the phase(s) detected in the four representative catalystsections analyzed by XRD. A dark field TEM image of Example 9 is shownin FIG. 7A; arrows denote regions where nanometer-scale pores areevident. A set of EDS element maps for C, Pt, Ni, and Ir, taken of thesame area as FIG. 7A, showing the spatial distribution of elements, isshown in FIG. 7B. A linear composition profile through the entirethickness of a catalyzed whisker detected by EDS, plotted as elementweight percentage, is shown in FIG. 8E. A linear composition profilethrough the entire thickness of a catalyzed whisker, calculated as Pt,Ni, and Ir mole fractions from the Pt, Ni, and Ir weight percentage datashown in FIG. 8E, is shown in FIG. 8F. H₂/Air polarization curves, takenbefore the AST (“BOT”) and after the AST (“AST”), are shown in FIG. 10C.

Comparative Example D

Comparative Example D was generally prepared as described forComparative Example A, except that independent single element Pt and Nitargets were used instead of a single alloy target, and the catalyst wassubsequently annealed. A single Pt layer with planar equivalentthickness of about 2.3 nm was first deposited onto the whiskers on MCTSfrom a pure Pt target (obtained from Materion, Mayfield Heights, Ohio).Next, a single Ni layer with planar equivalent thickness of about 3.9 nmwas deposited from a pure Ni target (obtained from Materion). The Pt andNi deposition processes were repeated several times, resulting in anareal loading of about 0.12 mg_(Pt)/cm². The targeted individual Pt andNi layer thicknesses were calculated to yield an overall composition of30 at. % Pt and 70 at. % Ni for the combined layers. Pt_(x)Ni_(y)catalysts deposited from individual single element Pt and Ni targets arereferred to as “multi target” (MT).

After deposition, the electrocatalyst was thermally annealed.Electrocatalyst on MCTS was placed into a quartz tube furnace (obtainedunder the trade designation “LINDBERG BLUE M” from Thermo ElectronCorporation, Waltham, Mass.) and heated to 430° C. under flowing H₂.After about a 20-minute temperature ramp, the catalyst was annealed forabout 3 hours at temperature, and then allowed to cool to roomtemperature over about a 3-hour period. After cooling to roomtemperature, the tube furnace was purged with nitrogen for about 15minutes to remove any remaining H₂, after which the catalyst onsubstrate was removed from the furnace. The annealed catalyst was thenfabricated into a CCM and as described for Example 1.

Comparative Example D was analyzed as described for Example 1. Loadingand composition information is in Table 7, below. Catalyst performanceand durability metrics are provided in Table 8, below. H₂/Airpolarization curves, taken before the AST (“BOT”) and after the AST(“After AST”), are shown in FIG. 11A.

TABLE 7 PtNi Ir Loading, mg/cm² Composition, at. % Pt:Ir ExampleDeposition Incorporation Pt Ni Ir PGM Pt Ni Ir Weight Ratio Comp. Ex. DMT none 0.122 0.086 0.000 0.122 30.0 70.0 0.0 Infinite Ex. 10 MT Bilayer0.0850 0.059 0.021 0.1059 28.1 64.9 7.0 4.0 Ex. 11 MT Bilayer 0.0850 NA0.021 0.1059 NA NA NA 4.0

TABLE 8 Ir H₂/Air Ir Content Specific Area ORR Mass Activity PerformanceExample Incorporation (mg/cm²) m²/g_(PGM) % Change A/mg_(PGM) % ChangeVolts Change Comp. Ex. D none 0 17.5 −46.5 0.327 −62.3 0.905 −0.050 Ex.10 Bilayer 0.021 12.9 +71.4 0.110 +49.6 0.838 +0.042 Ex. 11 Bilayer0.021 17.7 −1.4 0.183 −6.4 0.884 −0.004

Example 10 was generally prepared as described for Comparative ExampleD, except that Ir metal was also incorporated and the Pt and Ni loadingwas reduced. A single Pt layer with planar equivalent thickness of about1.6 nm was first deposited from a pure Pt target (obtained fromMaterion, Mayfield Heights, Ohio). Next, a single Ni layer with planarequivalent thickness of about 2.6 nm was deposited from a pure Ni target(obtained from Materion). Next, a single Ir layer with planar equivalentthickness of about 0.4 nm was deposited from a pure Ir target (obtainedfrom Materion). The Pt, Ni, and Ir deposition processes were repeatedseveral times, resulting in an areal loading of about 0.085 mg_(Pt)/cm².The catalyst was then annealed and fabricated into a CCM as describedfor Comparative Example D.

Example 10 was analyzed analogously to Comparative Example D. Loadingand composition information is in Table 7, above. Catalyst performanceand durability metrics are provided in Table 8, above. H₂/Airpolarization curves, taken before the AST (“BOT”) and after the AST(“After AST”), are shown in FIG. 11B.

Example 11

Example 11 was generally prepared as described for Example 10, exceptthat the catalyst was dealloyed after annealing, using the methoddescribed for Comparative Example C.

Example 11 was analyzed as described for Example 10, except that XRFcomposition analysis was not done after dealloying. Table 7, above,lists the Pt and Ni loadings and Pt:Ir weight ratio for Example 11,estimated from Example 10. Without being bound by theory, dealloyingpredominantly removes Ni from PtNiIr catalyst, so the Pt and Ni loadingsand Pt:Ir weight ratios are unchanged from the input material (Example10). Catalyst performance and durability metrics are provided in Table8, above. H₂/Air polarization curves, taken before the AST (“BOT”) andafter the AST (“After AST”), are shown in FIG. 11C.

Foreseeable modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this invention. This invention should not be restricted tothe embodiments that are set forth in this application for illustrativepurposes.

What is claimed is:
 1. A method comprising: providing an oxygenreduction catalyst material comprising PtNiIr, wherein there are layerscomprising platinum and nickel; and dealloying at least some layerscomprising platinum and nickel to remove nickel from at least one layerto provide a nanoporous oxygen reduction catalyst material having theformula Pt_(x)Ni_(y)Ir_(z), wherein x is in a range from 26.6 to 47.8, yis in a range from 48.7 to 70, and z is in a range from 1 to 11.4, andwherein the nanoporous oxygen reduction catalyst material has an exposediridium surface layer.
 2. The method of claim 1, further comprisingannealing the oxygen reduction catalyst material before dealloying. 3.The method of claim 1, further comprising depositing platinum and nickelfrom a target comprising platinum and nickel and depositing iridium froma target comprising iridium.
 4. The method of claim 1, furthercomprising depositing platinum from a target comprising platinum,depositing nickel from a target comprising nickel, and depositingiridium from a target comprising iridium.
 5. A method of making ananoporous oxygen reduction catalyst material comprising the formulaPt_(x)Ni_(y)Ir_(z), where x is in a range from 26.6 to 47.8, y is in arange from 48.7 to 70, and z is in a range from 1 to 11.4, and thenanoporous oxygen reduction catalyst material having an exposed iridiumsurface layer, the method comprising: depositing platinum and nickelfrom a target comprising platinum and nickel to provide a first layercomprising platinum and nickel; depositing a layer comprising iridiumfrom a target comprising iridium; repeating the preceding two steps, inorder, at least once to create a layered material; and dealloying atleast one layer comprising platinum and nickel to remove nickel from thelayer.
 6. The method of claim 5, further comprising annealing thelayered material before dealloying.